Ignore non-JSON stdout preamble in catch_discover_tests (#3162)

[ShivaPriyanShanmuga]

What

catch_discover_tests() runs the test executable with
--list-tests --reporter json and feeds the entire stdout into CMake's
string(JSON ...). If anything writes to stdout before main runs - most
commonly a third-party library logging from a static initializer - that text
precedes Catch2's JSON and the parser fails with:

string sub-command JSON failed parsing json string: * Line 1, Column 1
Syntax error: value, object or array expected.

Older versions used the plain --list-tests text output, which tolerated such
noise, so this is a regression people hit when upgrading.

Closes #3162.

How

Before parsing, strip everything before the first { (where Catch2's JSON
output begins) in extras/CatchAddTests.cmake. If no { is present at all,
fail with a clear error that includes the captured output.
This intentionally covers only the case described in the issue - output printed
before discovery starts (e.g. static-init constructors). It does not attempt
to handle output interleaved into the middle of the JSON.

Tests

Extended the existing CMakeHelper::DiscoverTests integration test:

• register-tests.cpp now prints to stdout from a static initializer, so the
  discovered executable reproduces the scenario.
• VerifyRegistration.py parses the executable's raw stdout directly, so it
  skips to the first { the same way the CMake script does.

Verified locally: the test fails on devel (JSON syntax error during
discovery) and passes with this change (5 tests matched).